The whispers mentioning Tim Southee and the New Zealand test vice-captaincy in the same sentence grow more audible by the day.

While no-one from New Zealand Cricket - or the Black Caps - has confirmed this potentially significant appointment, there has been no denial, either.

The man himself insists the role of Brendon McCullum's deputy has not been offered to him for the first test against South Africa in Cape Town on January 2.

However, the 24-year-old appears open to the prospect.

"It's up to Brendon and [coach] Mike [Hesson] to make that decision. I haven't heard anything about it, so I'll just have to wait and see," Southee said.

Kane Williamson, who captained the limited-overs team against West Indies this year when McCullum and former skipper Ross Taylor were absent, would be a strong contender, too.

Southee is the big mover in the ranks in recent months. He has ably assumed the leadership of the test bowling attack, with Chris Martin unable to make the 11, and appears highly rated by McCullum.

The new skipper, on the eve of New Zealand's tour of South Africa, said the test vice-captaincy was "still to be discussed".

"Tim is going really, really well at the moment, he's obviously bowling sensationally and he has been a leader in this team for a little while," McCullum said.

"There's Tim and there's Kane Williamson as well, who's got a great cricketing brain and is a lot more mature than his age would suggest. There's a number of other guys who have got some great leadership qualities as well."

McCullum said establishing a core leadership group was his priority in a largely young test squad.

"Age is irrelevant in that," McCullum said. "Certainly, just because you've played a long time doesn't necessarily make you a strong leader. There's some young guys in our team who I'll definitely look to for assistance."

Southee, who is due to become a father on Friday, was excused from the three Twenty20 internationals, which start in Durban on Saturday (NZ time).

He is in the form of his life, evidenced by his remarkable 156 off 130 balls, and 4-16 for Northern Districts on day one of the Plunket Shield match against Wellington at Karori Park on Monday.

In New Zealand's most recent three tests, in India and Sri Lanka, Southee took 20 wickets at 14.90, including 8-120 in the 167-run win in Colombo last month.

"You look around the bowlers and Chris Martin has done it for a number of years and done a great job and he still contributes a lot even though he's not playing. It's great to have him around the group and we understand he's not going to be there forever. It's a young bowling group and it's important we learn as much as we can from Tommy [Martin] while he's there.

"With Dan [Vettori] not there as well, it's important we all step up and take some responsibility."